Values Of Heat Conductivity Research Articles

Heat Flux Calibration Measurement Using the Noninteger System Identification Method for Cooled Surfaces

ACTIVE cooling mechanisms are used in many applications for thermal protection of surfaces in combustion chambers, rocket nozzles, gas turbine blades, and only recently structures of reentry vehicles [1–4]. The basic principle is to inject a cooling fluid close to the wall to reduce the wall heat load. In literature, the three different effects, i.e., film, transpiration, or effusion cooling, are named to occur in actively cooled structures. These cooling mechanisms, however, complicate the problem of the determination of a heat transfer coefficient to thewall and thus a calculation of the heatflux to the wall becomes challenging. Although transpiration cooling has been proven theoretically as an effective cooling mechanism by many investigators, its technological application became recently of particular interest with the invention of high-temperature porous material [2]. The progress inmaterial development allows a very new approach to high-temperature environments. From rocket engine combustion chambers and nozzles to turbine blades, from thermal protection systems for reentry vehicles to fusion chamberwalls, in all these modern high-temperature environments, transpiration cooling is currently in the focus of research to overcome the principle problem of exceeding the wall temperature limit [4–8]. Regenerative cooling is state of the art in rocket engines. Liquid fuel is used to cool the nozzle and the heated fuel is injected in the combustion chamber at higher temperature, which again increases the combustion efficiency. Results of numerical analysis show that the efficiency of regenerative cooling compared with transpiration cooling can lower the wall temperature by more than 30%. Transpiration cooling means that the fuel is injected through the wall into the combustion chamber or nozzle. This is advantageous because a layer of relatively cool fuel close to the wall protects the wall [5]. With the invention of high-temperature porous media for rocket engines, those numerically proved improvements could be verified by experimental tests [3]. For high-speed aircraft, material and cooling issues for both airframe and engine are the key elements which force the designer to limit the flight Mach number [9]. All these engineering problems face the fact that the heatflux at the surface is a design critical value yet difficult to measure. In the high heat flux regimes, a direct measurement is not applicable, since no sensor is available to measure directly at the surface. The only solution is therefore, a temperature measurement using in-depth temperature sensors. However, an inversemethod has to be applied to determine surface heat flux. In the case of a transpiration cooling environment, however, the often applied one-dimensional analytical calculation is prone to error: The thermocouple position is of particular importance in this approach and particularly in a porous material very difficult to know with sufficient precision. Moreover, the porosity has to be known, which complicates this analytical problem further because there is not only the dependency from heat capacity, heat conductivity (both values needed for the porous material), but also from thematerial specific parameter of porosity. A numerical approach lacks a precise information of porosity of the material which is impossible to know for a particular sensor system. Finally, for such complex materials, the thermophysical properties are not known and difficult to determine. A recent publication of Shi andWang presents a solution for this inverse problem [10]. Here, the accuracy of themethod again depends strongly on the accuracy of the information about the specific heat capacity, heat conductivity, and coolant temperature. In this paper, an approach based on the calibration of the real environment will be presented. For the analysis of the calibration measurement, the noninteger system identification (NISI) approach is applied to a transpiration cooled environment. Using basic calibration experiments, the transpiration cooling is inherently covered by the NISI calibration process. It is shown for the first time that the knowledge of the coolant mass flow is sufficient to determine the surface heat flux.

Simulation and Experimental Study on Temperature Distribution of Mass Steel Fiber Concrete

Temperature and elastic modulus of mass steel fiber concrete change as time in casting process. If not well handled, it is easy to crack in the process of construction. In order to hold the temperature distribution of mass high strength concrete in pouring process, we set up the model that meets with engineering characteristics. It is based on the partial differential equation of heat conduction and elastic modulus value from test and cement hydration heat of concrete on site. By contrast, the simulation results are in good agreement with the experimental results.

Thermal properties of semiconductor zinc oxide nanostructures

A combination of two methods — laser modulation and 3ω — has been used to determine the heat capacity, heat conductivity, and heat diffusivity of zinc oxide nanostructures. A significant difference between the thermal parameters of zinc oxide nanostructures grown by different technological methods has been revealed. It has been shown that the relatively low heat conductivity and heat diffusivity values of oxide zinc nanostructures are due to both the internal defects and the contact resistance between the film and its base — the substrate.

Anisotropic thermal conductivity and permeability of compacted expanded natural graphite

The anisotropic thermal conductivities and permeabilities are investigated for discs and plates of compacted expanded natural graphite. The measuring directions of heat conductivity and permeability are both parallel and perpendicular to the pressing direction of compacted samples. An unexpected phenomenon is found in that the thermal conductivity sometimes decreases as the density of the material increases, and this phenomenon only occurs for thermal conduction parallel to the compressing direction. The results also indicate that the direction perpendicular to the compression direction shows higher thermal conductive properties and permeability values. Both anisotropic thermal conductivities and permeabilities are strongly dependent on density. Analysis shows that as a type of porous material, the ENG yields layers under the effect of pressure, and their orientation influences the values of heat conductivity and permeability of the different samples.

Design of thermostatic tank and control system

Based on the isothermal forming technique of pressing and spinning, designed the precision formational equipment. Furthermore, auxiliary equipped the thermostatic oil tank through the heat conduction computation, the heat conductivity value’s analysis. at the same time using microcontroller digital logic operations, realization of the system high-precision automatic control. The result presents that the idea is reasonable, the device design is feasible and a value for promoting.

Thermal-mechanical properties of carbon nanotubes: molecular dynamics simulation

We determine single-wall carbon nanotube (SWCNT) thermal conductivity and tunable flattening dynamics at heat flux ranging from 0.01 to 0.3 W/m2 subject to different thermal loading of 5 to 50 K/nm, using a nonequilibrium molecular dynamics (MD) simulation with true carbon potentials. The numerical model adopts Morse bending, a harmonic cosine, and a torsion potential. The applied Nosé-Hoover thermostate describes atomic interactions taking place between the atoms. Hot and cold temperature reservoirs are respectively imposed on both computational domain sides to establish the temperature gradient along the carbon nanotube. Atoms at the interface exhibit transient behavior and undergo an exponential type decay with exerted temperature gradient. The thermal impact causes system fluctuation in the initial 3 ps leading to a transport region temperature as high as 600 K. The thermal relaxation process reduces impact energy influence after 30 ps and leads to Maxwell's distribution. Steady-state constant heat flux is observed after thermal equilibrium. Furthermore, the temperature curves show distinct high disturbance at initial time and linear distribution along the tube axial direction after steady state. Results suggest that thermal conductivity value increases with increasing CNTs subjected to thermal loading up to a temperature gradient of at least ~41.3 K/Å, representing thermal gradient convergence at heat conduction value 1258 W/mK. Simulation results yield precise understanding of nanoscale transient heat transfer characteristics in a single-wall carbon nanotube. Last, it is shown that given a thermal loading of sufficient intensity, the initial round cross section of the hot end of the nanotube transits through a series of triangular-like states to a flattened, rectangular configuration. As time elapses, the cross section oscillates between two fully perpendicular flattened states at a frequency that increases linearly with the intensity of the applied thermal load. (Cont'd.)

Global and local characterization of the thermal diffusivities of SiC f/SiC composites with infrared thermography and flash method

Two types of SiC f/SiC composites with different matrix quantities are prepared by CVR from pyrolyzed Carbon/resin composites. Experiments based on transient, space-resolved infrared thermography are developed; various assessment methods are implemented to measure simultaneously transverse and in-plane thermal diffusivities, globally and locally; the emphasis is set on the accuracy of the estimations. The material anisotropy is revealed and the influence of matrix volume fraction on the global thermal diffusivities is evaluated. Gradients of the properties are clearly visible in the samples, by use of the local analysis. The global heat conductivity values are discussed with respect to previous works.

The thermal and physical characteristics of the Gao-Guenie (H5) meteorite

Measurements of the bulk density, grain density, porosity, and magnetic susceptibility of 19 Gao-Guenie H5 chondrite meteorite samples are presented. We find average values of bulk density 〈 ρ bulk〉=3.46±0.07 g/cm 3, grain density 〈 ρ grain〉=3.53±0.08 g/cm 3, porosity 〈 P(%)〉=2.46±1.39, and bulk mass magnetic susceptibility 〈log χ〉=5.23±0.11. Measurements of the specific heat capacity for a 3.01-g Gao-Guenie sample, a 61.37-g Gao-Guenie sample, a 62.35-g Jilin H5 chondrite meteorite sample, and a 51.37-g Sikhote-Alin IIAB Iron meteorite sample are also presented. Temperature interpolation formula are further provided for the specific heat capacity, thermal conductivity, and thermal diffusivity of the 3.01-g Gao-Guenie sample in the temperature range 300< T (K)<800. We briefly review the possible effects of the newly deduced specific heat and thermal conductivity values on the ablation of meteoroids within the Earth's atmosphere, the modeling of asteroid interiors and the orbital evolution of meteoroids through the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect.

Salt Fog Evaluation of RTV SIR Coatings With Different Fillers

The performance of three commercially available room-temperature-vulcanized (RTV) silicone rubber (SIR) coatings was evaluated by means of salt fog testing. Leakage current and contact angle measurements were used in order to monitor the coatings' performance. The measurements indicated that the initial loss of hydrophobicity can be correlated to the surface discharges that develop on the material surface due to the droplets formed. In the case of the silica-filled coating, the considered los.s was observed earlier. Further, lower levels of surface activity and material deterioration were observed for the alumina-trihydrate (ATH)-filled coatings in comparison to the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> filled. The results can be correlated to the filler efficiency and especially to the thermal conductivity achieved. For the considered formulations, the silica-filled coating appeared to have the lowest value of heat conductivity. Finally, for the ATH-filled coatings, additional parameters, such as the particle size, must be considered.

Change in Physical Properties of Cotton Fabric Caused by Natural Indigo Dyeing

Physical properties of cotton fabric, which was dyed with natural indigo reduced by fermentation, were studied. Cotton fabric showed maximum value of tensile energy, WT, tensile strength and abrasion resistance at dye uptake of 44 mg/g. Mean of friction coefficient, MIU, showed a maximum at dye uptake of 11.6 mg/g and then decreased with an increase in dye uptake. XRD reflection proved that cotton fiber was dyed with natural indigo in aggregate state. Natural indigo dyeing gave cotton fabric smaller air permeability, smaller heat conductivity and smaller initial maximum value of heat flux than synthetic indigo dyeing with an increase in dyeing times. The cotton fabric dyed with natural indigo exhibited larger mechanical properties except for abrasion resistance than those with synthetic indigo reduced by hydrosulfite at dye uptake of about 45 mg/g. It was suggested that natural indigo dyeing changed the structure of cotton fiber in association with high-order structure of fiber.

Statistical analysis of fluctuations and noise-driven transport in particle-in-cell simulations of plasma turbulence

The problem of discrete particle noise has been studied based on direct fluctuation measurements from gyrokinetic particle-in-cell simulations of stable plasmas. From the statistical analysis of electrostatic potential time evolution, the space-time correlation function has been measured. Fluctuation spectra have been constructed and analyzed in detail. Noise-driven transport is calculated using the quasilinear expression for the diffusion coefficient and the obtained noise spectrum. The theoretical value of electron heat conductivity shows good agreement with that measured in the simulation. It has been shown that for the realistic parameters in actual turbulence simulations, the noise-driven transport depends linearly on the entropy of the system. This study makes it possible to estimate and subtract the noise contribution to the total transport during turbulence simulations.

Manufacture of high heat conductivity resistant clay bricks containing perlite

Different methods have been investigated for achieving heat insulation in the buildings. Manufacturing of high heat conductivity resistant construction materials is an important part of these research efforts. Perlite is an extremely useful material for heat insulation and 70% of the world reserves are located in Turkey. Nearly 65% of the perlite produced today is consumed by the construction industry. Its thermal, lightness, and acoustic insulation properties make perlite an excellent material to be used as lightweight aggregate in brick manufacturing. High heat resistant brick can be produced by adding perlite into the clay in conventional brick manufacturing. In this investigation perlite of Eskişehir region and clay were collated and fired to form high heat conductivity resistant material. Binding materials such as cement, gypsum, lime, bitumen and clay were used for manufacturing perlite brick. Bricks in standard sizes manufactured at different perlite–clay ratios and unit weight, compressive strength, volume reduction and heat conductivity values were obtained. Then the mixture with the best combination of the properties was determined and cost optimization was described. Results were examined according to combination properties, and specialties of perlite bricks were determined at various weights. As a result, the best mixture was determined as the one containing 30% perlite.

Temperatures of Coal Particle During Devolatilization in Fluidized Bed Combustion Reactor

The purpose of this study was to investigate the thermal behavior of coal during devolatilization in fluidized bed. Temperatures in the center of single coal particle were measured by thermocouple. Two coals were tested (brown coal Bogovina and lignite Kosovo), using dry coal particle, shaped into spherical form of diameters 7 and 10 mm, in temperature range from 300 to 850°C. Unsteady behavior of coal particle during heating and devolatilization in fluidized bed was described by a model that takes into account heat transfer between bed and particle surface, heat transfer through particle and an endothermic chemical reaction of first-order. Based on the mathematical model analysis and compared with experimental results, values of heat conductivity (λ c ) and heat capacity (C p ) of coal were determined. The best agreement was obtained for constant thermal properties, for brown coal λ c = 0.20 W/mK and C p = 1200 J/kgK and for lignite λ c = 0.17 W/mK and C p = 1100 J/kgK.

Acousto-mechanical and thermal properties of clotted blood

The efficacy of ultrasound-assisted thrombolysis as an adjunct treatment of ischemic stroke is being widely investigated. To determine the role of ultrasound hyperthermia in the process of blood clot disruption, the acousto-mechanical and thermal properties of clotted blood were measured in vitro, namely, density, speed of sound, frequency-dependent attenuation, specific heat, and thermal conductivity. The amplitude coefficient of attenuation of the clots was determined for 120 kHz, 1.0 MHz, and 3.5 MHz ultrasound at room temperature (20 +/- 2 degrees C). The attenuation coefficient ranged from 0.10 to 0.30 Np/cm in porcine clots and from 0.09 to 0.23 Np/cm in human clots. The experimentally determined values of specific heat and thermal conductivity for porcine clotted blood are (3.2 +/- 0.5) x 10(3) J/kg x K and 0.55 +/- 0.13 W/m x K, respectively, and for human clotted blood are (3.5 +/- 0.8) x 10(3) J/kg x K and 0.59 +/- 0.11 W/m x K, respectively. Measurements of the acousto-mechanical and thermal properties of clotted blood can be helpful in theoretical modeling of ultrasound hyperthermia in ultrasound-assisted thrombolysis and other high-intensity focused ultrasound applications.

Effect of variable thermal properties of the solid phase on composite solid propellant combustion

Effects of variable thermal properties (specific heat and thermal conductivity) of the solid phase on the combustion of composite solid propellants are studied analytically. Both steady and unsteady burning are considered. It is shown that the effects of variable thermal properties are small and can be accounted for by choosing proper average values of specific heat and thermal conductivity.

Thermal Properties of Sugarbeet Roots

Sugarbeet (Beta vulgaris L.) roots are subjected to heating and cooling during storage and processing 3iJld their response to heat transfer is dependent on thermal properties of the roots. Specific heat and thermal conductivity values are used in the design and modeling of ventilated storage of sugarbeet roots. Specific heat ofsugarbeet roots was measured using the method of indirect mixtures. A probe was constructed using a small diameter brass tube, type l' thermocouple wire, and a constantan wire acting a s heater to measure thermal conductivity of sngarbeet roots. The measured specific heat of sugarbeet roots (3.5464 kJ/kgK) was similar to the predicted specific heat values from Siebel's correlation and Reidel's calculation. The measured specific heat ofsugarbeet roots was similar to values for apple pomes and potato tubers. The thermal ctonductivity for frozen sugarbeet roots was twice that of unfrozen roots. Observed thermal conduc­ tivity values for sugarbeet roots were similar to -values reported for apple pomes and potato tubers.

An unsteady state method for the measurement of polymer thermal diffusivity: I. Development of a cell

In this work the development of a cell able to determine the thermal diffusivity of polymers in their fused or solid state, using an unsteady state technique, is presented. In this case a step down perturbation method was used because of its easy experimental setup. For each experimental run a temperature step down perturbation of 10 °C was applied, and the temperature evolution with time and position were registered and saved. Thermal diffusivities were then determined by regression using those data and a simple analytical model. The polymers used to test the cell were high density polyethylene and polypropylene (PP). For the first polymer, the thermal diffusivity was found to be 2.05×10 −7 m 2/s, which compare satisfactorily with a value of 1.97×10 −7 m 2/s calculated from reported values of density, heat capacity and heat conductivity. For the PP the value was found to be 7.08×10 −7 m 2/s with a positive deviation of 5.05% when compared with a computed value of 6.74×10 −7 m 2/s. Taking into account experimental errors, and the natural variations between different stocks of materials, the observed differences are acceptable.

A microscopic model for the thermal conductivity of a porous 380-aluminium alloy

The effective values of thermal diffusivity, specific heat, and thermal conductivity of a porous 380-aluminium alloy were investigated as a function of the volume percentage of porosity in the range 0%-10.35%. The samplesof the alloy were prepared by melting in a gas-fired furnace. The sample characterisation was carried out by means of differential calorimetry, photoacoustics, density measurements, and a computer image analysis. The thermal conductivity obtained on samples with the low degree of porosity is in good agreement with the data published for the solid 380-aluminium alloy. The measured thermal diffusivity of the porous samples shows only a weak dependence on porosity, whereas the thermal conductivity and the specific heat have an approximately linear decrease with the increase in the amount of porosity. To explain the experimental results, the existing phenomenological models were analysed; in addition, a new microscopic approach is proposed which takes into account the features of the motion of the electrons and phonons in a porous media. The microscopic model fits the experimental results well.

Free convection from a point source of heat, and heat transfer from spheres at small Grashof numbers

A numerical description, based on the Boussinesq equations, is given for the steady free convection flow due to a point source of heat and heated spheres. We begin with the non-dimensional formulation of the problem for the point source giving the numerical solution for a wide range of values of the Prandtl number, the remaining parameter. The analytical description of the temperature and flow fields close to the point source includes constants that are evaluated numerically and are used to obtain the flow field around heated spheres for small Grashof numbers, and the correction of the Nusselt number, due to free convection, from its heat conduction value. The numerical solution of the free convection problem over a sphere at moderate and small Grashof numbers is carried out for Pr=0.72 and 7. Based on the small and large Grashof number descriptions, a correlation expression is proposed for the laminar flow Nusselt number, covering all Grashof numbers.

INVESTIGATIONS INTO PROPERTIES OF MINERAL WOOL SLABS OF INCREASED STIFFNESS MANUFACTURED OF A LARGE—SIZE BEAM/PADIDINTO STANDUMO MINERALINĖS VATOS PLOKŠČIŲ, PAGAMINTŲ IŠ SUFORMUOTO STAMBIŲ GABARITŲ MASYVO, SAVYBĖS

The article presents the results of experimental investigations into non-uniformity of bulk density distribution, compressive strength, heat conductivity, and completion of binders polycondensation in slabs manufactured from a large-size mineral wool beam (its width B = 1000 mm and thickness H = 500 mm). The non-uniformity of bulk density was investigated as in a beam (for this purpose from different places of a beam there were cut samples with a thickness d = 120 mm), as in slabs (their length L = 1000 mm and width H = 500 mm). For such a purpose samples or slabs were cut after scheme (Fig 1) determining bulk density of each paralellepiped gi-j . The non-uniformity of bulk density was evaluated by values K H and K V , representing a relative departure from beam or slab bulk density gm. These values were calculated after formulae (1) and (2) where gi is the i-th vertical layer bulk thickness; gj is the j-th horizontal layer bulk thickness, n is number of vertical layers, m is number of horizontal layers. Variation of values K H and K V shows that relative departure from slab or beam bulk density does not exceed ± 20% (Fig 2). Comparing our experimental data with standard GOST 9573–82 requirements, one can see that compressive strength values of slabs s can exceed the standard one up to two times without increasing their bulk density (Fig 3). Better results are obtained when the content of binders is greater (within standard requirements). Also, it is possible to see that heat conductivity l because of structural changes of fibres distribution in a slab slightly increases (not more than 5%), when we are comparing this heat conductivity value with a maximally admitted standard one (Fig 4). Employment of this method allows the production of slabs, the optimal thickness of which has the ability to change from 6 to 500 mm. The binders polycondensation in such slabs is fully completed. Such slabs are suitable for insulating flat roofs, but their stiffness must be fastened by glueing them with stiff material.